A randomly nano-structured scattering layer for transparent organic light emitting diodes. - PDF Download Free (2024)

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A randomly nano-structured scattering layer for transparent organic light emitting diodes† Jin Woo Huh, Jin-Wook Shin, Doo-Hee Cho, Jaehyun Moon, Chul Woong Joo, Seung Koo Park, Joohyun Hwang, Nam Sung Cho, Jonghee Lee, Jun-Han Han, Hye Yong Chu and Jeong-Ik Lee* A random scattering layer (RSL) consisting of a random nano-structure (RNS) and a high refractive index planarization layer (HRI PL) is suggested and demonstrated as an efficient internal light-extracting layer for transparent organic light emitting diodes (TOLEDs). By introducing the RSL, a remarkable enhancement of 40% and 46% in external quantum efficiency (EQE) and luminous efficacy (LE) was achieved without causing deterioration in the transmittance. Additionally, with the use of the RSL, the

Received 19th March 2014 Accepted 5th July 2014

viewing angle dependency of EL spectra was reduced to a marginal degree. The results were interpreted

DOI: 10.1039/c4nr01520g

as the stronger influence of the scattering effect over the microcavity. The RSL can be applied widely in TOLEDs as an effective light-extracting layer for extracting the waveguide mode of confined light at the

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indium tin oxide (ITO)/OLED stack without introducing spectral changes in TOLEDs.

Introduction Practical out-coupling efficiency in transparent organic lightemitting diodes (TOLEDs), when the emission through only one side is counted, is lower than that of conventional bottom emissive OLEDs. Furthermore, even the total efficiency considered in both bottom and top sides of TOLEDs, at best, currently reaches only 70–80% of corresponding bottom emissive OLEDs. In addition, fundamentally, the light out-coupling efficiency for an OLED with a at glass substrate is limited to 20%.1,2 Therefore, improvement of the out-coupling efficiency in TOLEDs is a more serious issue than that in conventional OLEDs. In general, the out-coupling losses are caused by surface plasmon polaritons (SPPs) at a metal–organic interface, waveguide mode in an ITO/organic layer, and substrate mode in a glass substrate. Particularly, a signicant portion (60%) of generated light is conned within the substrate mode and the waveguide mode due to total internal reection (TIR).2 While loss of the substrate mode can easily be recovered by attaching a commercial micro-lens array or a light extraction lm on the surface of the substrate, the light extraction of the waveguide mode is much more challenging to achieve3–5 because it requires a modication of the internal structure inside the device. Hence, various designs and techniques for enhancing OLED Research Center, Electronics and Telecommunications Research Institute, Daejeon 305-700, Korea. E-mail: [emailprotected]; Fax: +82-42-860-5202; Tel: +8242-860-0826 † Electronic supplementary information (ESI) available: Simulation results of total (bottom and top) radiance of TOLEDs with the RSL depending on HTL and ETL thicknesses. See DOI: 10.1039/c4nr01520g

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light extraction of the waveguide mode have been proposed, including substrate surface modications,6,7 low-index grids,8 and Bragg diffraction gratings.9–11 As trials for enhancing light extraction in TOLEDs, Choi et al.12 suggested the periodically perforated WO3 layer to apply in TOLEDs for light extraction. Inspite of its effectiveness in improving outcoupling efficiency with optical clarity in the TOLEDs, the WO3 layer showed EL spectral changes at a specic wavelength satisfying the Bragg diffraction condition caused by the periodic structure. To overcome these changes in spectrum or CIE color coordinates, Kim et al.13 at the same group introduced WO3 nanoislands and they exhibited induced invariance of spectrum changes and no specic angular dependency. However, they focused on bottom-emissive OLEDs. Research to achieve transmittance for its application to TOLEDs still remains unresolved. Recently, Chang et al.14,15 proposed a nanoparticle-based scattering layer (NPSL) to stabilize the spectral dependency on the viewing angle of the outcoupled light. Although the NPSL has been shown to be highly effective as a light extraction structure for white OLEDs,14 TOLEDs with NPSL called NPSL-based bi-directional OLEDs have very low transmittance due to its very high haze of 85%,15 which is not suitable for TOLEDs. In this work, we suggest a random nano-structure (RNS) as a new approach to improve both out-coupling efficiency and spectral angular independency without compromising transmittance in TOLEDs. In order to achieve our goal, rst, an RNS was fabricated. Second, light extraction in TOLEDs equipped with the RNS was optimized by adjusting the microcavity length. In this course, the thicknesses of the hole transport layer (HTL) and the electron transport layer (ETL) were varied.

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Microcavity and scattering effects of the OLED-stack structure were investigated by varying the HTL thickness using both computer simulations (2- and 3-dimension) and experiments. As a result, total efficiency was remarkably enhanced without introducing spectral-change over the viewing angle and no signicant decrease in transmittance. The mechanism of light extraction in the suggested structure of TOLEDs was also investigated.

Experimental details Random scattering layer (RSL) and the RSL embedded TOLED Fig. 1a and b show the TOLED structures we used. One was equipped with an RNS and the other not. The RNS cannot be applied directly in the TOLED because its rough surface deteriorates electrical characteristics of the OLED device. Thus, we coated a high refractive index planarization layer (HRI PL) composed of TiO2 on the RNS as shown in Fig. 1c. We call the layer composed of a RNS and a HRI PL as a random scattering layer (RSL). The preparation of the RSL has been previously reported.16 The scattering layer consists of irregular nano-sized pillars which have heights of 350 nm and diameters of 200– 500 nm. An 1 mm thick planarization layer was formed on the nano-pillars. The planarization layer has a high refractive index (n) of 2.02 at 550 nm. It is important to choose a planarization material which has higher n than ITO, otherwise a big fraction of the generated light will undergo total internal reection. The TOLED we fabricated has green-colored emission with its main peak at l ¼ 540 nm. The stack of the TOLED we fabricated is as follows; indium tin oxide (ITO) (100 nm)/1,1-bis[(di-4-tolylamino)phenyl] cyclohexane (TAPC) (100, 160, and 230 nm)/ 4,40 ,400 -tris(N-carbazolyl)-triphenylamine (TCTA) (5 nm)/phosphorescent emission layer (10 nm)/2,6-bis(3-(carbazol-9-yl)phenyl) pyridine (DCzPPy) (10 nm)/1,3-bis(3,5-di-pyrid-3-ylphenyl)benzene (BmPyPB) (40 nm)/LiF (1 nm)/Al (1.5 nm)/Ag (15 nm)/TAPC (120 nm). Based on the results of our previous report,17,18 we introduced TAPC of 120 nm as a capping layer (CL) on the Ag cathode to maximize total efficiency by enhancing reectance of the cathode. The fabrication processes have been described in our previous studies.19 We basically fabricated the TOLEDs with an emitting area of 1.5 1.5 mm2 to measure current–voltage–luminescence (I–V–L) characteristics of the devices. In order to capture the scattering feature, the luminance values were measured as a function of viewing angle. Later, the

Fig. 1 Structures of the TOLEDs (a) without and (b) with the RSL. (c) SEM images of the RNS (top) and cross-section of the RSL (bottom).

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values were integrated to obtain an overall performance for a given TOLED. In this course, we have used devices with an active area of 7 10 mm2 for collecting angularly integrated values and measured the emission as a function of angle. External quantum efficiency (EQE) and luminous efficacy (LE) were calculated by integrating angular and spectrally resolved emissions. The electroluminescence spectrum was measured using a Minolta CS-2000. The I–V–L characteristics were obtained with a source/measure unit (Keithley 238) and a Minolta CS-100. Transmittance was measured using an UV-visible spectrophotometer (U-3501, Hitachi). Additionally, transmission haze and total transmittance were measured using a haze meter (hazegard plus, BYK Additives & Instruments). Computer simulations To deduce experimental conditions for investigating the microcavity effects in the TOLED, we performed simulations using a commercial OLED optical simulator, SimOLED.20,21 The scattering effects of the RSL were also investigated using a three dimensional (3D) nite-difference time-domain (FDTD) simulation method.22,23 The size of the computational domain was 8 mm 8 mm 10 mm, where the last dimension refers to the direction perpendicular to the OLED layers. The spatial resolution in the FDTD calculation was 20 nm. In order to effectively mimic the OLED light source, we have constructed an emitting layer which consists of distributed dipole sources. The dipole sources have orthogonal x-, y-, z-polarizations and a central wavelength of 540 nm. For RNS, numerous nano-pillars with a height of 350 nm and a diameter range of 200–500 nm were randomly distributed on the substrate. Based on the microstructural observations, the spacing between nano-pillars was chosen to have a range of 50–500 nm. From the FDTD simulations, we obtained far eld (E2-intensity) proles and their integrated values, which are being plotted on a hemispherical surface detector. In order to obtain realistic simulation results, we used all measured optical constants (n, k) of organic materials, which were measured using an ellipsometer (M-2000D, J. A. Woollam Co.).

Results and discussion Optical effect of RSL In order to investigate the microcavity effect, the scattering layer was treated as a hom*ogeneous single layer without any scattering structure (Fig. 2a and b). Subwavelength features were used to obtain an effective refractive index.24,25 This TOLED was compared with the reference TOLED which has no RSL between the glass and the ITO. In the reference device, the microcavity forms between the Ag cathode and the ITO anode. However, in TOLEDs with the RSL, due to the high optical contrast between the RSL and the glass, microcavity takes place between the Ag cathode and the RSL. Fig. 2c shows simulated results of the dependence of the HTL thickness on the cavity effect. The thickness of ETL was xed at 40 nm based on our preliminary simulation results (see ESI†) and the thickness of the HTL was varied in a range of

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Schematic diagrams of the TOLED structures (a) without and (b) with the RSL, and (c) their simulated results as a function of HTL thickness. (Arrows designate the structure where microcavity forms.)

Fig. 2

0–300 nm in simulations. The total radiances in the bottom and top direction in both devices show periodic oscillation depending on the HTL thickness due to resonance. The radiance has maximum and minimum at a specic HTL thickness: in TOLEDs with the RSL, the highest total radiance was obtained in the thickness range of 60–80 nm (the rst order) and 220–240 nm (the second order); the lowest was observed near 160 nm. The period in TOLEDs with the RSL is 170 nm and similar to that in the reference device, but its phase is shied behind the reference TOLED. This phase change might be induced from the change of reection at the bottom side in the cavity structure of the RSL-embedded device.26 In Fig. 2c, a remarkable change to be worthy of notice is that the resonance in the TOLED with the RSL is considerably weakened relative to that of the reference TOLED, while strong cavity is sustained in the reference device. This simulated result can be interpreted by the change in the microcavity on optical length and coherence in the device structure. The total optical length of the microcavity (ML) in OLEDs with multi-layer structures is given27,28 by ML ¼

P nidi + |(4ml)/(4p)|

(1)

where ni and di are the refractive index and thickness of the i-th lm layer. 4m is the phase shi at the Ag cathode reector and l is the free-space wavelength. In eqn (1), the second term, which is the effective penetration depth into the top metal mirror, is usually small in comparison to the rst term.28 Hence ML can be approximated as the rst term, the sum of the optical thicknesses of the layers. Parameters relevant to our work and calculated ML are listed in Table 1. The table shows that the microcavity lengths in the TOLEDs without and with the RSL are about 0.3–0.8 mm and about 2.2–2.8 mm, respectively. In addition, to investigate the optical effect of the planarization layer without the RNS, we also calculated ML, of which results are summarized in Table 1. The calculation result shows that the ML in the TOLED equipped with only HRI PL is more or less the same to that TOLED equipped with the RSL. These results strongly suggest that the ML in the device with the RSL is dominated by the HRI PL. If PL is applied only without the RNS in OLEDs, the cavity length increases monotonically due to its similarity in refractive index to that of ITO, additionally, internal absorption is expected to increase through the thick layer of the PL.

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Coherence in a typical OLED structure maintains to some degree since the total lm thickness in the structure is not longer than the wavelength of emissive light. However, in the RSL-embedded OLED structure, we should consider the coherence length29 because the total thickness exceeds the visible wavelength range. Moreover, the resonance of light waves can only arise over the coherence length in the cavity structure. The coherence length is approximated by l2/Dl (ref. 29 and 30) where l and Dl are the light wavelength and the full-width at half maximum, respectively. From the formula, using l ¼ 540 nm and Dl ¼ 80 nm in emitted light, the coherence length of the emitted light in two devices was calculated to be about 3.7 mm. This means that the microcavity length in the device with the RSL or only the HRI PL is comparable to the coherence length, thus the degree of microcavity effects is considerably lessened compared to that of the reference device. Accordingly, the overall OLED light output is expected to be proportional to the cavity extent.26,31,32 This cavity effect of the RSL in the device is conrmed in the simulations (Fig. 2c).

Performance of TOLEDs with RSL In order to verify the optical effect on the HTL thickness and the light extraction effect on the RSL in TOLEDs, we fabricated the green TOLED device with and without the RSL. In particular, we paid attention to the inuence of the microcavity design. Based on simulation results, we chose three thicknesses of HTLs, which correspond to maximum (230 nm), minimum (160 nm) and midpoint (100 nm) peaks in the radiance variation. To avoid the occurrence of internal-short circuit, which can be caused by protruding parts of nano-pillars, we intentionally chose the HTL thicker than 100 nm. The transmittance of the ITO substrate with the RSL is shown in Fig. 3a. The transmittance is 60% at 500 nm and reaches at 70–80% above the region of 550 nm. In this substrate, haze was measured to be 15%, which is due to light scattering by the nano-sized pillars. When the substrate with the RSL is embedded in the TOLEDs (Fig. 3b), the transmittance of the device maintains 55–60% above the region of 500 nm although the transmittance in the region below 500 nm is relatively low. The low transmittance in the wavelength region shorter than 500 nm might be caused by the diffusion of light due to maximization of scattering because the wavelength is comparable to the size of nano-pillars. Total transmittance (TT), sum of direct (Fig. 3b) and diffuse transmittances, was measured to be up to 70%. Though scattering attributed to the RSL in TOLEDs brings about haze and decreases direct transmittance, it contributes to diffuse transmittance and increases TT aer all. This strongly indicates that the RSL can spread light over wide wavelength range without serious absorption loss. Such a feature is of prime importance in the eld of internal light extraction. The pictures in Fig. 3b show actual images of TOLEDs. In both cases, the letter “T” is clearly discernible without blur. Fig. 4 shows the bottom and top angular distributions of luminance as a function of HTL thickness. The luminance was measured at a constant current density of 2 mA cm2. In the case of the device without the RSL, the effect of microcavity can

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Refractive index and thickness of each layer in the TOLED structures with and without the RSL, and with only HRI PL

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ni di (mm) Materials

Refractive index (ni)

Thickness (di) (mm)

Without RSL

With RSL

With HRI PL

1 2 3

Organic ITO HRI PL

0.11–0.64 0.20

0.11–0.64 0.20 2.02

RNS + HRI PL

0.065–0.365 (HTL: 0–0.3) 0.100 1.000 0.650 0.350

0.11–0.64 0.20

4 P Total ( ) ML

1.76 1.98 2.02 2.02 1.76

0.31–0.84

1.31 0.62 2.24–2.77

Fig. 3 Transmittance of (a) the substrate with the RSL and (b) the TOLEDs with and without the RSL.

be clearly observed. The microcavity designed OLED with a HTL thickness of 230 nm showed higher bottom and top luminance than those with 100 nm and 160 nm HTL thicknesses in the normal direction (q ¼ 0 ). However, in the case of the RSL embedded devices, the luminance proles were very similar to each other irrespective of the HTL thickness. In addition, insertion of the RSL was observed to have an effect of enhancing luminance uniformly without preference to specic viewing angle. We attribute those features to the strong scattering effect of the RSL and alleviated microcavity effect. This observation is particularly evident in the bottom emissions rather than top emission. Thus, more signicant improvement is achieved in the bottom-side emission. The difference in bottom and top emissions will be discussed using the EL spectra. Our approach suggests an effective method for extracting the waveguide mode of conned light at the ITO/OLED stack and spreading out the

2.33–2.86

light in a diffusive fashion. The process and mechanism for light extraction of waveguide mode will be described in the following. Evaluated total external quantum efficiencies (EQEs) and luminous efficacies (LEs) as shown in Fig. 5 support these discussions. With the RSL, it was observed that the total EQE increased to 22–24% with an enhancement of 27–40%, and also the total LE improved up to 55l m W1 with an enhancement of 28–46% (Fig. 5a). It is noticeable that the EQEs and LEs of the RSL embedded TOLEDs improved to a very similar level with rather weak dependency on the HTL thickness. However the microcavity effect is signicant in the reference TOLEDs as mentioned above. These observations were clearly reproduced in the FDTD simulations on the far eld radiation (Fig. 5b). Oscillatory behavior in the far eld radiation in the reference TOLEDs is almost attened out in the RSL embedded TOLED. Agreement between the simulated and measured data can be observed in their EQE and LE dependency on the HTL thickness. From the practical view point, the dominancy of scattering and weak dependency of microcavity is very useful in light

(a) EQE and LE and (b) FDTD-simulated data depending on the HTL thickness in the TOLEDs with and without the RSL. (c) J–V–L characteristics in the TOLEDs with the RSL (at an HTL thickness of 160 nm). Fig. 5

Fig. 4 Angular distribution of luminance in (a) bottom and (b) top

directions.

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extraction design. When we use the RSL in the TOLEDs, the TOLED with a 160 nm HTL corresponding to the thickness of the minimum radiance shows almost the same EQE and LE values as the TOLED with optimized HTL thickness corresponding to the maximum radiance. Accordingly, the highest light extraction enhancement was achieved in the device with a HTL thickness of 160 nm. The total increments in EQE and LE were 40% and 46%, respectively. Table 2 summarizes measured total EQE and LE values of each TOLED. Fig. 5c shows the J–V–L (current density–voltage–luminance) characteristics of TOLEDs with the RSL in the case of a HTL thickness of 160 nm. The L–V plot shows that the TOLED with the RSL showed distinct improvement in the luminance of bottom and top sides compared to the reference TOLED. However, in the J–V plot, two devices showed almost identical current density–voltage characteristics. Referring to J–V–L measurements, we can draw a conclusion that the RSL induced luminance enhancement is purely optical not electrical. The effect of introducing the RSL on the electro-luminescence (EL) spectra is shown in Fig. 6. The EL spectra were collected from the normal viewing angle (q ¼ 0 ). All EL spectra in bottom and top sides were normalized. Arrow spans in the graph of Fig. 6a designate minimum and maximum of Full Width at Half Maximum (FWHM) in spectra of TOLEDs with different HTL thicknesses. When the RSL is absent, the FWHM changes considerably as the HTL thickness changes. With the introduction of the RSL, the difference in the FWHMs diminishes compared to the reference TOLED, which may be attributed to dominancy of scattering effect over the optical microcavity effect. In the bottom side, presumably due to the scattering dominancy, the FWHM tends to be slightly wider because introduction of the RSL causes microcavity effects to abate. Meanwhile peakdistortions or -shis were observed to appear in top side EL spectra. Particularly at the HTL thickness of 230 nm where the cavity effect was maximized from the simulated result, the spectral peak position and shape were changed considerably. Recalling the measurement results of Fig. 4, the EL spectra in the top side are understood as a combination of scattering and microcavity effects. The corresponding two factors may be described as follows. First, the scattering effect in the top side is originated only from the light component reected from the RSL which is located at the bottom side. Second, in our structure, the CL of 120 nm plays a role in maximizing reectance at the top side,13 leading to strengthen microcavity effects. Therefore, the contribution of scattering is weaker than that of the microcavity in the top emission due to the above two factors.

Table 2

Fig. 6c illustrates our interpretation on the optical phenomenon inside the TOLED with the RSL and describes the light extraction mechanism. In the gure, we assume identical emission of DEo and UEo, which originate from the emissive organic layer. When the emitted light of DEo encounters the RSL, scattering toward the bottom and top directions occurs. In the case of the bottom direction, the scattered light components toward the substrate primarily contribute to the scattering effect: DEo(SD) (scattered light of DEo toward the bottom side), UI ER(SD) (scattered light of UIER which is the reected light of U Eo from the lower part of CL), and UIIER(SD) (scattered light of UII ER which is the reected light of UEo from the upper part of CL). This scattering effect at the bottom side can be understood as a random diffraction event which facilitates the outcoupling of the conned light at the glass/ITO interface and ITO-organic stacks. However, if light is initially scattered toward the top side by diffusive reection from the RSL (DEo(SU)), the reected light of DEo(SU) from the CL (see dotted lines in Fig. 6c) indirectly contributes to light extraction of the waveguide mode for bottom emission. Meanwhile, for the top side emission, the scattering effect in the top side originates only from the diffusively reected light (DEo(SU).) from the RSL. Although there may be additional sequential scattering toward the top side by reection, the scattering effect is negligible. These overall processes indicate that the scattering effect in the bottom side is stronger than that in the top side. Conversely, in the top emission, the microcavity effect is more apparent. The larger enhancement in the bottom-side emission arises from the direct extraction of waveguide mode at the ITO– organic stack due to strong scattering. The relatively small enhancement of the top side emission is due to the randomly reected light components from the RSL layer. Furthermore, the high reectance of the CL gives boost to the bottom emission and yields a bottom/top emission ratio of 5.17 The CL enhances microcavity, which may cause distortion in the EL spectra. Our results in Fig. 6a and b show that the RSL has a function of relieving spectral dependency by averaging the light traveling paths, resulting in suppressed microcavity effects and spectral stability. Fig. 6d shows the Commission Internationale de l'Eclairage (CIE) coordinates as a function of viewing angle. The HTL thickness is 230 nm, which corresponds to a strong microcavity condition. The TOLEDs with the RSL have lower variation in the CIE coordinates. This trend is more obvious in the bottom emission than top emission. As the angle changes from 0 to 70 , CIE (Dx, Dy) of the reference TOLED in bottom and top sides are (0.038, 0.028) and (0.044, 0.317), respectively. In the

External quantum efficiency and power efficiency of the TOLED with and without the RSL Luminous efficacy (lm W1)

External quantum efficiency (%) HTL (nm) (ETL40 nm)

Without RSL

With RSL

Enhancement

Without RSL

With RSL

Enhancement

100 160 230

18.2 16.3 18.0

23.1 22.8 24.0

26.9% 39.9% 33.3%

38.4 37.3 43.1

55.1 54.5 55.0

43.5% 46.1% 27.6%

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(q ¼ 0 ) CIE coordinates between with and without the RSL were achieved within D(0.02, 0.02) (bottom) and D(0.05, 0.04) (top) in all HTL thicknesses. With the use of the HTL thickness of 160 nm, the color change due to introduction of the RSL was extremely small. The CIE coordinates vary negligibly as D(0.005, 0.007) and D(0.01, 0.02) in bottom and top emissions, respectively.

Conclusions In conclusion, we fabricated transparent OLEDs (TOLEDs) with an internal nano-structured random scattering layer (RSL) and demonstrated the RSL as an internal light-extracting layer to improve both efficiency and spectral viewing angle independency. The average transmittance of the TOLEDs with the RSL was 57% and the haze was 15%, which were comparable to TOLEDs without the RSL. The use of the RSL showed remarkable enhancements of 40% and 46% in external quantum efficiency (EQE) and luminous efficacy (LE), even the haze of TOLEDs with the RSL was not high. The EQE and LE reached 24% and 55 lm W1, respectively. Optical effects in the internal structure of TOLEDs were also investigated as a function of HTL thickness with simulations and experiments. Without the RSL, the out-coupling of TOLEDs is governed by the microcavity effects dominantly. When the RSL was incorporated into the TOLED, the scattering dominates the internal optical effects. This is in accordance with the simulation results, which veried to weaken the microcavity effect due to the RSL. Experimentally, because of the scattering dominancy and mitigated microcavity, it was possible to achieve nearly identical efficiencies with distinguished enhancement in the RSL embedded TOLEDs irrespective of the HTL thickness. In addition, with the use of the RSL it was possible to reduce the viewing angle dependency of EL spectra to a marginal degree. Based on our results, we expect the random nanostructured scattering layer has great potential as an internal light-extracting layer to enhance total efficiency without introducing spectral changes in TOLEDs.

Acknowledgements

Fig. 6 Comparison of the emission between the TOLEDs (a) without and (b) with the RSL in bottom and top sides, respectively. (c) Schematics of emissive light components in the TOLEDs with the RSL. (d) CIE color coordinates vs. viewing angle in the TOLEDs without and with the RSL (under the HTL thickness of 230 nm).

RSL equipped device, as the angle changes from 0 to 70 , the CIE (Dx, Dy) in bottom and top sides is (0.009, 0.009) and (0.003, 0.029), respectively. In addition, the introduction of the RSL scarcely changes the color coordinate. The differences in the normal direction

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This work was supported by the IT R&D program of Ministry of Trade, Industry and Energy (MOTIE)/Korea Evaluation Institute of Industrial Technology (KEIT), Rep. of Korea (KI002068, 10041062, and 10044412, Development of Eco-Emotional OLED Flat-Panel Lighting, Development of Fundamental Technology for Light Extraction of OLED and Development of OLED panels using graphene) and the Basic R&D program of ETRI, and IT R&D program of Development of Key Technology for Interactive Smart OLED Lighting, which is a part of ETRI Internal Research Fund from Ministry of Science, ICT and Future Planning (MSIP).

Notes and references 1 C. F. Madigan, M. H. Lu and J. C. Sturm, Appl. Phys. Lett., 2000, 76, 1650–1652.

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